1. Field of the Invention:
This invention relates to a mobile radio communication system which includes a plurality of mobile stations, a plurality of radio base stations communicating with the mobile stations using a plurality of channels of different frequencies and having specific radio areas, and an operation maintenance station which supervises and controls an operation situation of the system and wherein the same channel can be allocated to a plurality of radio base stations and an automatic frequency allocation method performed in the system.
2. Description of the Prior Art:
In a mobile radio communication system, each radio base station having a specific radio area called a cell must be allocated channels for all the transmitter-receivers in the cell from the limited number of frequencies (channels) authorized for use. In using this limited number of frequencies to accommodate the largest number of subscribers and process the maximum number of cells, there is a method of allocating the same channel in the same system to different radio base station.
In such a case, when the same channel in the same system is used simultaneously by a plurality of proximate radio base stations, the same frequency used by another station causes interference and the quality of a cell deteriorates. Due to this disturbance, in system designing, allocation of channels (radio frequency allocation) must be performed so that the same channel can be utilized efficiently and interference between channels is minimized.
In recent years, mobile radio communication systems have become remarkably large in scale, and systems have been constructed which consist of hundreds of base stations. In radio frequency allocation for such a large scale mobile radio communication system, it is very difficult to design a precise system in a short time based on the desk judgment of a designer, because the number of factors to be taken into consideration in designing and the amount of data to be handled are very large.
In this regard, a method of performing radio frequency allocation by computer simulation has been proposed. For example, a method is available which seeks to allocate radio frequencies by simulation under fixed assumptive conditions, and also an attempt to enter actual system operation situations and radio speech quality data into a simulation system to raise the simulation accuracy has been performed.
Meanwhile, as a frequency allocation method which does not use a simulation system, an automatic frequency allocation method has been proposed wherein communication carrier frequencies used by other base stations are monitored to detect whether or not a communication carrier frequency is used commonly by a base station and the other base stations and, if a communication carrier frequency which is used commonly by any of the other base stations is detected, then the communication carrier frequency of the base station is re-selected thereby to change the communication carrier frequency of the base station to prevent interference with the other base stations (refer to Japanese Patent Laid-Open Application No. Heisei 4-68925).
However, with the method described above wherein radio frequency allocation is performed by simulation, since a radio frequency allocation obtained is a result of the simulation obtained under the fixed assumptive conditions and does not reflect an actual operation situation, it is not an optimum radio frequency allocation.
Also where an actual system operation situation and radio speech quality data are entered into a simulation system, there is a problem in that the amount of data to be collected is very large and much time is required for their collection and an operation situation of the system which varies from time to time cannot be followed up. Further, there is another problem in that, since measurement data include errors and fluctuations for each measurement, the radio communication situations obtained from results of the measurements are not fixed and simulation cannot achieve convergence to an optimum solution or a real solution. Consequently, the method described above still fails to introduce a normally optimum radio frequency allocation following up a situation of the system.
Further, where simulation is used, since it does not involve autonomous judgment of whether or not a set radio frequency allocation is optimum, it is required for a system operator to supervise the system operation situation from time to time and discriminate from the operation situation and a radio speech quality situation whether or not the radio frequency allocation need be changed.
With the automatic frequency allocation method disclosed in Japanese Patent Laid-Open Application No. Heisei 4-68925, although frequency allocation can be performed automatically on an actual system so that interference with a different base station may not occur, since the same channel is not used by different stations in the system, the method is not suitable for a mobile radio communication system which re-allocates the same channel in the same system.